Re: Help with AP GTO on Parallax 125

Mike Mah

Thanks for the advice, everyone.

What is the three star alignment procedure? I don't recall seeing
this in the manual.

Mike Mah

--- In, "ericj" <ericj@m...> wrote:
Hi Mike:

I just recently went through this with my 900 GTO mount so can
offer a
couple of suggestions.

This is how I tried to align the mount and scope on Polaris, as
suggested in
the AP manual that comes with the mount. After setting up the mount
setting the Dec setting circle to 90 degrees I would look through
the polar
alignment sight hole in the center of the polar axis to align on
Then I would add the counterweight shaft and counterweights, and
finally the
tube assembly. Each time I did this though I noted that OTA would
be offset
slightly to the left or west of Polaris.

At first I thought that I had somehow shifted the mount a little
when adding
the counterweights and tube assembly, so would just "pick up" (more
nudge) the mount and scope until Polaris was aligned through the
scope. But
I found that even after using the two star alignment method I would
not end
up anywhere near the deep-sky object that I was trying to find
using the GTO

So finally I looked through the polar alignment sight hole and to
surprise Polaris was no where to be seen, even though it was still
in scope. At this point I figured the problem had to be that the
rings were
not orthogonal with the mount. So off came the OTA and after
checking the
rings realized they were fine. I then removed the ribbed mounting
plate and
examined it to see if there could be a problem with it. It was fine

My next thought was that since I had used the used the scope and
rings on my
home made Dob mount for the past four years that somehow I warped
the rings
slightly (I rotate the rings on their sides and attach side
bearings to
them; a photo of how they look is on my web page at, and click on Telescopes). I
couldn't be
sure if this was a problem or not, and figured the only way to be
sure was
to find another pair of AP mounting rings that would not be warped.
Since I
have only had my 5.1" AP for less then a year I figured the
likelihood that
these rings would be warped as the rings for the 7.1" were was

So off came the 7.1" and rings, and on went the 5.1" and rings.
problem. Finally, I decided to look down the polar alignment sight
hole from
the front to see if it looked aligned correctly. It didn't. With
the setting
circle to 90 degrees the Dec axis seemed off to one side. I aligned
it by
eye so that it appeared to be aligned correctly, and noted that the
setting circle read 87 degrees. Back on went the 5.1" with rings,
and this
time with the mount aligned on Polaris the scope was aligned on
Polaris as

Did the two star alignment method, but although the scope slewed
closer to
the deep-sky object it still was off. So I finally tried using
three star to
align the mount: Polaris, Vega, and Regulus. This time, the scope
slewed to
M101 correctly, and with the high contrast of the 5.1" the galaxy
was sharp,
well defined and surprisingly bright. Tried a few more objects and
results were the same.

At this point, even though it was a during the work week, and it
was almost
midnight, I couldn't resist the temptation of not using the 7.1".
So off
came the 5.1" and on went the 7.1". This time I was even more
careful when
aligning the mount, so I got to the point that I just left a 16mm
eyepiece in the MaxBright when slewing from object to object and
they were
often very close to the center of the FOV, which in the 16mm is
around half
a degree or so. I finally got to bed around 2 AM, but it was worth
it, even
though I dead tired the next day a work.

So I have a couple of suggestions for you. First, check to make
sure that
the rings and OTA is orthogonal with the mount.

Second, check to see if, when looking down the front of the polar
sight hole on your mount, if the Dec axis is centered or not. If
not, you
cold have the same problem that I do. I have sent an e-mail to Marj
mentioned it so she is aware of the situation. Whether yours has
the same
problem or not I am not sure, but you can check.

Third, when you are doing you alignment do not use the N-S-E-W
buttons to
recenter the star if it is off to one side. Use the altitude and
adjusters on your mount for this.

Fourth, try using three star rather then two.

Hope this helps. For what it is worth, I have found the 900 to be a
fine and stable mount. I got into astronomy in the early 1970's,
and got
quite use to star hopping to find deep-sky objects. When computer
telescopes first came out I did not think I would ever want one.
since I have now had a chance to use the GTO feature I realize that
are many deep-sky objects, like some of the fainter NGC or IC
objects, that
will be much easier to find and observe now. So I look forward to
many years
of finding and observing with my AP scopes and mounts.

Clear Skies,

Eric Jamison

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